1. Technical Field
The present invention relates to an image formation device which, with plural image-forming sections, forms respective single-color images on an image-bearing body and superposes the plural single-color images on the image-bearing body to form an output image.
2. Related Art
An example of a tandem-type color image formation device for outputting color images is provided with respective image formation sections for each of the colors yellow (Y), magenta (M), cyan (C) and black (K). At each of these image formation sections, a respective toner image of the color Y, M, C or K is formed on a photosensitive body, such as a photosensitive drum or the like. A color image (output image) is formed by sequentially transferring these toner images onto a common image creation region of an intermediate transfer body, such as an intermediate transfer belt or the like, and overlaying the toner images. The output image that has been formed on this intermediate transfer body is transferred and fixed onto a recording medium, such as recording paper or the like, and outputted to outside the device.
Now, with a tandem-type color image formation device as described above, positional offsets when the toner images of the respective colors are superposed on the intermediate transfer body may be discerned as color registration error image faults (defects) in color images, or as reductions in image quality which result from color registration errors. Accordingly, a technology has been proposed (see Japanese Patent No. 2,765,606) in which, at predetermined times such as immediately after a power supply is turned on or prior to the commencement of image formation, pattern images for detection of color registration errors (i.e., registration control patterns) are sequentially formed at the intermediate transfer body by the image formation sections of the respective colors. Positions of these registration control patterns are respectively detected by an optical sensor or the like. On the basis of the detection values, image-writing timings (exposure commencement timings) of images onto the photosensitive drums by the image formation sections are feedback-controlled. Thus, color registration errors between the toner images of the respective colors are eliminated.
However, with a tandem-type color image formation device as described above, periods in which it is possible to feedback-control timings of commencement of exposure onto the photosensitive drums by the image formation sections, in accordance with registration control pattern detection values, are limited to periods in which no effect will be exerted on image quality, such as, for example: a period immediately after power to the device is turned on; a period prior to formation of an output image; a period in which a non-image creation region (an inter-image region) between plural image creation regions (page regions) formed at the intermediate transfer body is passing an image formation section which is the object of feedback control; and suchlike. Therefore, in a case of forming an image at a long strip of continuous paper, if the image has a large image size in a sub-scanning direction, that is, if there is no margin to the image along a length dimension, color registration errors between the single-color images may gradually progressively increase during image formation, and image quality near a trailing end of an output image may be lower than near a leading end.
Furthermore, if a color registration error in an output image is detected during the formation of an image which is long along a sub scanning direction, correcting the color registration error (i.e., a positional offset of the toner images), by altering a timing of writing of an image by an image formation section that is an object of feedback control, during formation of the image, may be considered. However, if a correction amount applied to a toner image is large, a location at which the correction is applied may be discerned in the output image as an obvious defect, and there may be a deterioration in image quality.